Conventionally, subsea hydrocarbon production equipment is connected to floating storage and process or control facilities by a pipe or set of pipes called risers. Risers have taken many forms since they were first used. This invention relates to multiple pipe risers where a central pipe is surrounded by other smaller pipes or tubes. This type of riser can be in two forms, which are commonly referred to in the industry as Integral or Non-integral.
When risers are run from a surface rig they are typically made up of lengths of pipe about 50 ft. long. These lengths of pipe, referred to as joints, are joined together and lowered to the sea bed. It is the method of connecting the riser joints together and running them to the sea floor that introduces the label of Integral or Non-integral.
An integral riser is where a riser joint, i.e. a length of riser, is made with the central pipe and the surrounding pipes or tubes permanently joined together. When one joint of riser is attached to another joint of riser all the individual pipes are stabbed and joined together at the same time. This has the advantage of speed in joining and running the riser. It also means that when the riser is supported or tensioned at the surface only a single attachment is needed. There is also a serious disadvantage. Because the outer tubes are rigidly attached to the central pipe the complete riser acts as a one-piece structural member. When the riser bends, which is a normal operating state, the outer tubes take the highest load. In many cases it is not practical to make the outer tubes and their supports strong enough. Alternately, slip joints can be used but this introduces fretting and wear problems.
Non-integral risers are risers where the outer tubes are not rigidly attached to each other or the central pipe. The central pipe usually has guides at regular intervals along its length to hold the outer tubes in the correct lateral location. Typically the central pipe is run first by itself and locked onto the equipment subsea. The outer tubes are then run through the guides, one at a time, and locked to the equipment subsea. Each outer tube is tensioned separately from the surface and acts independently of the others. This arrangement also has the added benefit of being able to retrieve individual tubes if a fault develops without interfering with the rest of the riser.